Steel pipes of martensitic stainless steel like API 13Cr-steel has excellent corrosion in a CO2-containing atmosphere, and hence they are mainly used in oil well applications such as tubing and casing for use in excavation of oil wells. Martensitic stainless steel is hardened by quenching from a temperature in the austenite region (at a temperature equal to or above the Ac1 point of the steel) to form a martensitic structure. Therefore, it is normally subjected to final heat treatment for hardening after hot working.
However, the high hardenability of a martensitic stainless steel may cause martensitic transformation of the steel even while it is allowed to cool in air after hot working such as pipe formation, and in some cases cracks develop particularly in those portions to which an impact has been applied during handling of the product. This phenomenon which is referred to as delayed fracture suddenly takes place after a certain period of time has passed from hot working. Therefore, for hot working of martensitic stainless steel, it is necessary to prevent the occurrence of delayed fracture during the period after hot working and prior to heat treatment for hardening.
In the manufacture of martensitic stainless steel pipes, a common countermeasure against delayed fracture is to limit the length of time from the completion of pipe formation up to the start of heat treatment for hardening by quenching. To do so, shortly after pipe formation, the resulting pipe must be subjected to heat treatment to provide the steel with sufficient strength by quenching. However, limiting the time from pipe formation until heat treatment sometimes makes it necessary to frequently change the heat treatment temperature during operation, leading to a decrease in manufacturing efficiency.
JP 2004-43935A described a martensitic stainless seamless pipe with suppressed delayed fracture by a technique based on restriction of the amount of effective dissolved C and N (which is defined below) to 0.45 or less. However, the amount of effective dissolved C and N is determined by the composition of a steel, and when an appropriate steel composition is selected by considering other properties such as strength and toughness, there are cases that the amount of effective dissolved C and N exceeds 0.45. Therefore, this technique cannot be said to be perfect for prevention of delayed fracture.